Probing Heterogeneous Charge Distributions at the α-Al₂O₃(0001)/H₂O Interface

Area of Science:

• Surface Science
• Physical Chemistry
• Spectroscopy

Background:

• Insulating mineral oxides exhibit localized and heterogeneous surface charges due to protonation/deprotonation.
• Understanding these heterogeneous charges is crucial for various industrial, electrochemical, and geochemical processes.

Purpose of the Study:

• To use potassium thiocyanate as a molecular probe to investigate the heterogeneous interfacial electrostatic potential at the α-Al2O3(0001)/H2O interface.
• To spectroscopically measure local potentials associated with heterogeneously charged surfaces for the first time.

Main Methods:

• Vibrational sum frequency generation (vSFG) spectroscopy using the Stark active C≡N stretch of thiocyanate.
• Tracking changes in electrostatic potential via Stark shifts of the thiocyanate vibrational frequency.
• Density functional theory-molecular dynamics (DFT-MD) simulations to understand the origin of the vSFG signal.

Main Results:

• Simultaneous measurement of vSFG response from thiocyanate ions at charged and neutral surface sites.
• Assignment of local potentials of +308 mV (pH 4) and -154 mV (pH 10) to charged aluminol groups.
• Identification of distinct orientations of thiocyanate anions based on surface site charge and adsorption behavior (Langmuir isotherms).

Conclusions:

• This work presents the first spectroscopic measurement of local potentials on a heterogeneously charged surface.
• The findings provide vital insights into the behavior of interfacial water and ion adsorption on mineral oxides.
• The methodology offers a powerful tool for in situ probing of local charge evolution at complex interfaces.